Fujita-san, thank you for taking time! At Mon, 31 Aug 2020 19:10:39 +0900, Etsuro Fujita <etsuro.fuj...@gmail.com> wrote in > On Mon, Aug 31, 2020 at 6:20 PM Etsuro Fujita <etsuro.fuj...@gmail.com> wrote: > > * I know your patch is a POC one, but one concern about it (and > > Horiguchi-san's patch set) is concurrent data fetches by multiple > > foreign scan nodes using the same connection in the case of > > postgres_fdw. Here is an example causing an error: > > > select * from pt1, pt2 where pt2.a = 't22' or pt2.a = 't23'; > > ERROR: another command is already in progress > > CONTEXT: remote SQL command: DECLARE c4 CURSOR FOR > > SELECT a, b FROM public.t22 WHERE (((a = 't22'::text) OR (a = 't23'::text))) > > > (Horiguchi-san’s patch set doesn't work for this query either, causing > > the same error. Though, it looks like he intended to handle cases > > like this by a queuing system added to postgres_fdw to process such > > concurrent data fetches.) > > I was wrong here; Horiguchi-san's patch set works well for this query. > Maybe I did something wrong when testing his patch set. Sorry for > that.
Yeah. postgresIterateForeignScan calls vacate_connection() to make the underlying connection available if a server connection is busy with another remote query. The mechanism is backed by a waiting queue (add_async_waiter, move_to_next_waiter, remove_async_node). regards. -- Kyotaro Horiguchi NTT Open Source Software Center
